About two-thirds of the population have the variant of the serotonin gene (5-HTT) that increases vulnerability to depression, said the researchers at Radboud University Nijmegen in the Netherlands.

They analyzed five years of data collected from 428 families with two adolescent children each. The youngsters provided yearly information about their behavior and depressive symptoms.

The study found a strong association between marijuana use and an increase in depressive symptoms among those with the gene variant.

"The effect is robust. It still remains, even if you take into account a series of other variables that could cause the effect, such as smoking behavior, alcohol use, upbringing, personality and socioeconomic status," the researchers said in a university release.

"Some people might think that young people with a disposition for depression would start smoking cannabis as a form of self-medication, and that the presence of depressive symptoms is therefore the cause of cannabis use. However, in the longer term that is definitely not the case. Although the immediate effect of cannabis may be pleasant and cause a feeling of euphoria, in the longer term we observe that cannabis use leads to an increase in depressive symptoms in young people with this specific genotype," the researchers said.